This work is intended to determine whether ultrasound, in the medical diagnostic dose range, affects differentiating tissues. The forelimbs of newts will be amputated and exposed to ultrasound at various stages during regeneration, especially during the period of differentiation of tissues that will constitute the regenerate. The regenerated tissues will be examined grossly and histologically for differences between them and regenerated limbs not exposed to ultrasound. An attempt will be made to quantify ultrasound dose received by the exposed tissues.